#include <bits/stdc++.h>
using namespace std;
#define endl '\n'
#define ll long long
#define all(x) (x).begin(), (x).end()
int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
int n;
cin >> n;
vector<ll> v(n);
for (int i = 0; i < n; i++) cin >> v[i];
vector<ll> ansL(n), ansR(n);
for (int i = 1; i < n; i++) ansL[i] = ansL[i - 1] + max(0LL, v[i - 1] - v[i] + 1);
for (int i = n - 2; i >= 0; i--) ansR[i] = ansR[i + 1] + max(0LL, v[i + 1] - v[i] + 1);
ll ans = LLONG_MAX;
for (int i = 0; i < n; i++) ans = min(ans, max(ansL[i], ansR[i]));
cout << ans;
}
